This invention relates to a system and method for controlling the spindle of an electric motor, and more particularly to a system and method for controlling the spindle of a motor that rotates the platter of a disk drive.
Controlling the speed at which the platter of a disk drive rotates is very important, particularly as storage densities increase and platter size decreases. Thus, in a microdrive—i.e., a drive having a platter diameter of about 1 inch or less—even a small error in angular position resulting from an error in speed control may result in an incorrect sector being read or written. It is therefore a nominal goal to determine disk speed to within 0.01%.
Position, and therefore speed, of a disk drive platter is commonly determined by detecting the back electromagnetic field (back EMF) generated when one of the motor poles passes one of the stator poles. For example, it is typical for a disk drive motor to have six poles, so that each pole-pair interaction theoretically signifies 60° of motor rotation. However, in practice, it is difficult during manufacturing to accurately position the poles. Therefore, in practice, some sets of adjacent poles may be closer together than 60°, and other sets of adjacent poles may be further apart than 60°. These offsets may be slight, but may be enough to prevent achieving the desired 0.01% accuracy.
It therefore would be desirable to be able to more accurately control the speed of a motor, particularly in a disk drive, notwithstanding inaccurate pole placement.